The present invention relates generally to signal processing, and more specifically to techniques for processing signals recovered from a Fourier transform spectrometer.
A Fourier transform spectrometer typically includes an interferometer into which are directed an infrared beam to be analyzed and a monochromatic beam that provides a position reference. The interferometer has a fixed mirror and a movable mirror. In rapid scanning, the movable mirror is driven at a nominally constant velocity over a portion of its travel; in step scanning, the movable mirror is moved intermittently. Each of the input beams is split at a beam splitter with one portion traveling a path that causes it to reflect from the fixed mirror and another portion traveling a path that causes it to reflect from the movable mirror. The portions of each beam recombine at the beam splitter, and the recombined beams are directed to appropriate detectors.
The optical interference between the two beam portions causes the intensity of the monochromatic beam and each frequency component of the infrared beam to vary as a function of the component's optical frequency and the mirror position. The detector output represents the superposition of these components and, when sampled at regular distance intervals, provides an interferogram whose Fourier transform yields the desired spectrum.
In a rapid scan interferometer, when the mirror is moved at a constant speed, the monochromatic beam provides a nominally sinusoidal reference signal whose zero crossings occur each time the moving mirror travels an additional one quarter of the reference wavelength (i.e., for each half wavelength change of retardation). The data acquisition electronics are triggered on some of these zero crossings to provide regularly sampled values for the interferogram. With the appropriate choice of mirror velocity, the output signal can be made to fall within a convenient range of modulation frequencies, as for example in the audio range.
In a step scan interferometer, the movable mirror is moved from one reference point to the next and then stopped, at which point an intensity measurement is typically made. The sequence is then repeated until the desired interferogram has been acquired.
It is also known in the art to phase modulate the IR signal in a step-scanning interferometer. Phase modulation is a technique wherein a signal is applied to either the fixed or the moving mirror to dither the optical path length at each desired retardation. This is typically by an amount corresponding to .+-.90.degree. of phase shift of the shortest wavelength in the spectral range of interest (103.degree. is optimum). The phase modulation signal is typically sinusoidal; however, other more complex signals--both periodic and aperiodic--may be suitably employed. The infrared detector signal is passed through a demodulator such as a lock-in amplifier to detect the signal level at the dither frequency. While the shortest wavelength is modulated by almost 100%, the longer wavelengths are modulated to a lesser degree. The output of the lock-in amplifier at a given retardation value provides a measure of the derivative of the interferometer detector signal at that retardation.
A large transient signal is generated when the moving mirror in a step-scanning interferometer moves from one reference location to another. The large transient signal can introduce a substantial error in the measurement. Conceivably, this transient can be disconnected from the demodulator by a switch, but the switch generates another transient when it closes even after the step transient has died down. Hence, the solution employed in the current art is to wait a relatively long time after the mirror has been moved to a new position (i.e., several multiples of the time constant characterizing the lock-in amplifier) to allow the transient to settle before taking a measurement. This approach, however, waste valuable measurement time.